1. Field of the Invention
The present invention relates generally to a 3D shape measuring apparatus for acquiring interference patterns using an interferometer and measuring the shape of a three-dimensional (3D) object, and, more particularly, to a fast 3D shape measuring apparatus and method that is capable of acquiring a plurality of interference patterns within a short period of time, thereby enabling fast shape measurement.
2. Description of the Related Art
In general, an interferometer divides source light into a measurement beam and a reference beam using a beam splitter, causes the beams to be incident on a test surface and a reference mirror, respectively, and combines reflected beams using the beam splitter, thus creating an interference pattern including the height and shape information of the object to be measured. This interference pattern is input to a computer, which is a data processing unit, through respective pixels of a Charge-Coupled Device (CCD) camera in an area array in the form of digital values corresponding to the optical intensities of the interference pattern. Various methods ranging from the Carre's method, proposed by a French person Carre' in 1963, to the R+1 bucket, proposed in 1993, have been proposed as methods of extracting the height and shape information of a test surface using input digital values. These methods are each configured to acquire the height and shape information of an object to be measured from several interference patterns using a measurement algorithm. The basic principle of the methods is to acquire the height and shape information of a test surface by processing digital values corresponding to several interference patterns that are obtained by changing the optical path of source light in such a way as to move the reference mirror at specific regular phase intervals (for example, π/2, 2π/3 or the like) within one wavelength (=λ, 0˜π) of the source light. In this case, a piezoelectric actuator PZT is used to change the location of the reference mirror. The piezoelectric actuator requires settling time during which the piezoelectric actuator is driven to move the reference mirror, and a plurality of interference patterns cannot be acquired until the settling time has elapsed. Here, the settling time is the time that it takes to stably reach a desired location until setting is completed when the piezoelectric actuator is driven to move the reference mirror, and is the time that it takes to perform setting attributable to the movement of the reference mirror and that ranges from the time at which the piezoelectric actuator starts to be driven to the time at which the piezoelectric actuator completes driving.
The above relationship will be described in detail below with reference to FIG. 1.
FIG. 1 is a graph showing the relationship between moving distance and time when the piezoelectric actuator is driven.
Referring to FIG. 1, the relationship between moving distance and time when voltage is applied and the piezoelectric actuator is driven is shown, and the horizontal axis of FIG. 1 denotes time t and the vertical axis thereof denotes moving distance d when the piezoelectric actuator is driven.
When voltage is applied, the piezoelectric actuator is driven, starts to be moved and is moved over the maximum distance in the maximum overshoot interval (which occurs at tmax), and the moving distance is reduced and reaches a point in a stable-state interval ds. The settling time ts is terminated at the time at which the piezoelectric actuator enters a stable state, and image capture starts after the settling time. The settling time during which movement over a specific distance is completed is denoted by ts in the graph. That is, it can be seen that it takes settling time ts for the piezoelectric actuator to start to be driven and complete driving.
The graph of FIG. 1 shows the control characteristics of a typical piezoelectric actuator when the piezoelectric actuator is driven. The terms ‘maximum’, ‘settling time’ and ‘stable state’ are terms that are widely used in the general control field.
That is, the driving of the piezoelectric actuator is determined to have been completed when driving voltage is applied, the piezoelectric actuator is driven, settling time ts has elapsed and a stable-state interval ds is reached after the settling time ts, and then an image capture signal for acquiring an image is transmitted.
Accordingly, since settling time, during which the piezoelectric actuator is driven to move the reference mirror in order to acquire a plurality of interference patterns, is required, each settling time is required whenever each of the interference patterns is acquired, so that it is impossible to perform fast shape measurement.